Method for the electrolytic conditioning of metal tubes

ABSTRACT

The surfaces of metal tubing are dissolved and conditioned by electrolytic means. Direct electrical contact between the power source and tube, and the problems associated therewith, is avoided by concentrically disposing the tube between two relatively inert electrodes. A first, inner electrode is axially disposed within the tube, and the tube itself is axially disposed within a second, outer electrode. The electrodes are connected to opposite polarities of the power source. Therefore, depending on the polarity of the electrodes, the current enters one surface of tube from the electrolyte and exits to the electrolyte from the other surface thereof. If current is considered to flow from positive to negative, then dissolution will occur at the surface where current exits to the electrolyte. Superior removal of defects is achieved by reversing the polarity every 15-60 seconds.

United States Patent [191 Bengel et al.

[4 1 Sept. 16, 1975 METHOD FOR THE ELECTROLYTIC CONDITIONING OF NIETAL TUBES Inventors: Thomas G. Bengel, Pittsburgh;

Richard L. Sallo, Greensburg; Charles D. Stricker, Monroeville, all

of Pa.

[73] Assignee: United States Steel Corporation, Pittsburgh, Pa.

[22] Filed: June 13, 1973 [21] Appl. No: 369,643

[52] US. Cl. 204/129.43; 204/129.1; 204/129.5; 204/272 [51] Int. Cl. C25F 3/00; C25F 5/00 [58] Field of Search 204/129.43, 129.1, 272, 204/269, 270, 268, 129.5

[56] References Cited UNITED STATES PATENTS 820,113 5/1906 Hinkson 204/272 899,226 9/1908 Lutz 204/272 902,892 11/1908 Lutz 204/272 1,262,045 4/1918 Hcrrcshoff, Jr. 204/268 1,267,120 5/1918 Rudolf 204/268 1,267,141 5/1918 Stout 204/268 1,982,009 1 1/1934 McKinney ct a]. 1 204/272 2,412,186 12/1946 Whitehouse et al. 204/l29.1 2,706,175 4/1955 Licharz 204/272 2,764,540 9/1956 Farin et al. 204/272 2,934,631 4/1960 lmalis et al. 204/129.43 3,065,153 11/1962 Hough et a1. 204/272 3,104,221 9/1963 Hill 204/272 3,338,807 8/1967 Clifford..... 204/129.5 3,429,787 2/1969 Weinreich 204/272 Primary Examiner-John H. Mack Assistant Examiner-W. A. Langel Attorney, Agent, or Firm-Arthur J. Greif 5 7 ABSTRACT The surfaces of metal tubing are dissolved and conditioned by electrolytic means. Direct electrical contact between the power source and tube, and the problems associated therewith, is avoided by concentrically disposing the tube between two relatively inert elec trodes. A first, inner electrode is axially disposed within the tube, and the tube itself is axially disposed within a second, outer electrode. The electrodes are connected to opposite polarities of the power source. Therefore, depending on the polarity of the electrodes, the current enters one surface of tube from the electrolyte and exits to the electrolyte from the other surface thereof. 1f current is considered to flow from positive to negative, then dissolution will occur at the surface where current exits to the electrolyte. Superior removal of defects is achieved by reversing the polarity every 15-60 seconds.

4 Claims, 1 Drawing Figure METHOD FOR THE ELECTROLYTIC CONDITIONING OF METAL TUBES This invention relates to a method for the electrolytic conditioning of metal pipes and tubes and is particularly concerned with electrolytic machining to remove surface defects thereof. I

It is well known that metal tubing can be conditioned, i.e. polished or machined by electrolytic methods. In such prior art processes, through the use of a relatively inert, auxiliary electrode, an anodic potential is impressed upon the tube, causing it to dissolve at a rapid rate. Thus, if it is desired to electrohone the inside surface of the tube, the auxiliary electrode will generally take the form of a cylinder axially disposed within the tube. On the other hand, if the outside surface of the tube is to be dissolved, the tube itself will be axially disposed within a larger diameter, tubular cathode. It is similarly well known that such processes have generally been unsatisfactory in that (a) they have not yielded a uniform surface, and (b) they have been excessively costly and time consuming. In attempts to correct these deficiencies the art has resorted to, for example, electrolytic cells utilizing (i) specific cathode configurations, (ii) traveling cathodes, and (iii) electrolytes under high pressure. Although such attempts have been successful in producing a satisfactory surface, their generally complicated nature has actually added to the costs involved. The deficiencies of such conventional methods have been especially apparent in the electroconditioning of hot-pierced stainless steel tubing.

Seamless stainless-steel tubing is manufactured by hot piercing stainless rounds, followed by cold finishing. However, before performing the cold finishing operation, it is first necessary that an acceptable surface be produced by removing all roughness and surface imperfections such as drawn-in scale and inclusions, the latter being inherently produced by the hot piercing operation. In a conventional, non-electrolytic method, these defects are removed by a batch pickling operation followed by first grinding the outside surface and then honing the inside surface. It is this latter honing procedure which is especially costly. Because of the exactitude required in such a procedure, the tube must first be rounded. Rounding the tube requires pointing the end, lubricating and cold drawing. The pointed end is then cut off and the tube finally cleaned in preparation for honing. Thus, not only is such a process time consuming, but it is wasteful of metal, as well. In view thereof, it is clear why a major portion of the art relating to electroconditioning of tubing is specifically related to stainless steel tubing.

Because of the relatively poor conductivity of stainless steel, it has been particularly difficult to effect an even distribution of current along the tube, especially in long tube lengths. Further difficulties are encountered in the use of current densities sufficiently high to achieve practical rates of material removal. With conventional electroconditioning, the use of high current densities has resulted in severe 1 R heating of tube and arcing between the tube and the auxiliary cathode. These problems have, to a great extent, resulted from the methods generally employed in getting the current to the tubing. Thus, it has been found that if the current enters and exits the tubing through the electrolyte, then very high current densities can be employed without encountering the above problems, and nevertheless achieve uniform conditioning.

It is therefore a principal object of this invention to provide a method for supplying current to the tube through the electrolyte.

It is a further object of this; invention to provide a method whereby current may be evenly distributed along the surface of the tube.

It is yet another object of this invention to provide a method whereby current densities significantly higher than have heretofore been applied, may be employed.

Another object of this invention is to provide a method which is capable of equivalently and substantially simultaneously conditioning both the interior and exterior surface of a tube.

Yet another object of this invention is to provide a method whereby the tubes being conditioned may be readily slipped in and removed from the conditioning cell, without the necessity for connecting and disconnecting of metallic contacts.

Other objects and advantages of the invention will be apparent from the following description, when read in conjunction with the appended claims and the accompanying drawings, in which:

The figure is a schematic drawing embodying the essential features of an apparatus for carrying out the method of this invention.

With respect to the figure, the electrolytic cell consists of a cylindrical outer electrode 11, and a concentrically centered inner electrode 12. Both electrodes extend the full length of the longest metal tube 13 which is to be conditioned. As illustrated, various points of connection 14, may be provided to overcome possible IR drops in the length of the cathode. Electrolyte is pumped from a reservoir, not shown, into manifolds 15a and 15b, which feed electrolyte (depicted by arrows) at a high velocity, sufficient to permit passage of the requisite high current density. The electrolyte overflows through port 16 and is returned to the reservoir. The tube 13 being conditioned is concentrically positioned between the inner and outer electrodes, by means of an insulated sleeve support 17, resting on base 18.

In its specific application to the electroconditioning of stainless steel tubing, the outer electrode segments may be made of lead, while the inner electrode may be composed of a copper bar sheathed in lead. Conventional electropolishing electrolytes (eg in parts by volume, 38 parts I-I PO 29 parts -60 Be H SO 33 parts H O) may be employed. However, an electrolyte containing from about 20 to 60 wt. preferably 30-50%, H 80 was found to produce excellent surface uniformity when operated at temperatures of to F, preferably l20150F, with the passage of from 2 to 15 amps/m preferably 5-10 amps/in of current. This latter electrolyte is especially preferred since it is capable of passing high current densities (and therefore shorter processing cycles) while minimizing the chemical and electrochemical erosion of the lead electrodes.

Electrochemical processes which achieve electrochemical action without direct connection of the workpiece to the source of EMF are well known. See, for example, U.S. Pat. No. 3,338,809 wherein the electrolyzing current both enters and exits the wire through the electrolyte. As an aid to understanding such electrochemical action it should be noted that anodic dissolution will occur at that portion of the workpiece where current (taken as flowing from +to exits to the electrolyte. With this in mind, the operation of the cell (FIG. 1) will be described for the electrohoning of the inside of the tube. To achieve such an action, the outer electrode, 11, is connected to the positive pole of the rectifier (EMF source) while the inner electrode is connected to the negative pole. The electrolyzing current will then flow substantially radially from the outer auxiliary electrode through the first annular segment of the electrolyte to the inner auxiliary electrode. With the circuit connected in this manner, the dissolution rate of the inner surface of the tube will be increased, while the dissolution rate of the outer surface will be decreased (when compared with steady state dissolution with no applied EMF). It may therefore be seen that by simply reversing the polarity, the instant method is capable of conditioning the outer tube surface only, or of concommitantly conditioning both the inner and outer surfaces. With respect to this latter capability, as it applied to stainless steel tubing, it was found that the utilization of. polarity reversal provided even further benefits. In a number of trail runs residual scale was only partly removed on surfaces which were anodically polarized during the entire dissolution period. It was therefore necessary, to thoroughly remove all traces of scale, e.g. by a pickling operation, prior to such anodic electroconditioning. However, when the tube surfaces were electroconditioned by reversing the polarity every 30 seconds (i.e. a 60 second cycle) the scale was thor oughly removed. To determine the optimum polarityreversal cycle, stainless-steel panels were electroconditioned (at a current density of 6.9 amplin for a total time of 20 minutes in each case, employing reversal times of 0.25, 0.5, l, 5 and minutes. The shorter times (one minute and below) not only provided enhanced scale removal and better leveling of defects, but exhibited somewhat improved current efficiency as well. Further experiments indicated that this improved efficiency may be attributed to a reduction of the polarization of the metal surface. It is therefore desirable that stainless steel tubes be electroconditioned by reversing the polarity of the tube every to 60 seconds, with a time of about 30 seconds being most preferable. If it were necessary to dissolve a greater portion of a particular surface, then a reversing procedure could be employed in which that surface was anodic for a greater time. For example, the inside surface would be dissolved to a greater extent by making it anodic for a time 30 seconds and cathodic for a time of seconds; i.e. a total cycle of 50 seconds.

It should be understood that while the illustrative example above was primarily concerned with the electroconditioning of stainless steel tubing, that the process is clearly applicable to metal tubing in general. Similarly, for purposes of this invention, the term tubing is not limited to hollow conduits of circular cross section. Thus, the process is similarly applicable to hol low conduits of, for example, oval or polyganol cross sections. In the latter instance, if uniform dissolution of a tube surface is desired, it would then be preferable that the inner and outer electrodes have a somewhat conforming shape. That is, in the electroconditioning of an oval tube, it would be preferable to axially dispose theworkpiece within an oval shaped outer electrode and similarly axially dispose an oval inner electrode within the tube undergoing conditioning.

In general, the electrodes would be composed of a material which is relatively insoluble in the electrolyte being employed. Electrolytes which are conventionally employed for electromachining or electropolishing of metal surfaces may be employed in the instant process. However, since the instant method permits the use of considerably higher current densities, the added advantages of the instant invention will be realized by employing electrolytes of extremely high conductivity, i.e. electrolytes which permit the passage of the desired high current densities. As current density is increased, the region adjacent the electrode surface (generally the cathode) becomes increasingly depleted in the ions (generally H) required to support electrochemical reactions. A condition is eventually reached, known as the limiting diffusion current density, which represents the maximum rate of reduction for a particular system. This limiting current is governed by the concentration and temperature of the electrolyte, and most importantly by the degree of agitation of the solution. Here again, the decided advantage of the instant invention in permitting the use of current densities significantly higher than conventional electroconditioning, will only be realized if the agitation, concentration and temperature of the electrolyte are sufficient to permit the passage of such higher current densities.

We claim:

1. A method for the electroconditioning of a stainless steel tube, whichcomprises,

axially disposing said stainless tube within an outer,

tube-like electrode composed of a relatively insoluble material and axially disposing within said stainless tube, an inner, cylindrical electrode composed of a relatively insoluble material, said stainless tube being electrically insulated from said inner and outer electrodes, immersing said stainless tube and said inner and outer electrodes in an electrolyte consisting essentially of 20 to 60 wt. percent I-I SO maintained at a temperature of to 175F,

establishing a potential difference between said inner and outer electrodes, of a magnitude sufficient to effect the passage of a current density of 2 to 15 amperes per square inch, while causing said electrolyte to flow in the annular spaces between said electrodes and said stainless tube at a rate sufficient to support said current density, whereby the current is caused to pass substantially radially between the inner and outer electrodes through said tube and the electrolyte in the annular spaces between said electrodes and said tube, and the surface of said tube facing the more negative of said electrodes will be dissolved and conditioned.

2. The-method of claim 1, wherein said H 80 concentration is between 30 and 50 percent, said temperature is between and F and said current density is between 5 and 10 amperes per square inch.

3. The method of claim 2, wherein the polarity of said inner and outer electrodes is reversed every 10 to 60 seconds.

4. The method of claim 1, wherein the polarity of said inner and outer electrodes is reversed every 10 to 60 

1. A METHOD FOR THE ELECTROCONDITIONING OF A STAINLESS STEEL TUBE, WHICH COMPRISES, AXIALLY DISPOSING SAID STAINLESS TUBE WITHIN AN OUTER, TUBELIKE ELECTRODE COMPOSED OF A RELATIVELY INSOLUBLE MATERIAL AND AXIALLY DISPOSING WITHIN SAID STAINLESS TUBE, AN INNER, CYLINDERICAL ELECTRODE COMPOSED OF A RELATIVELY INSOUBLE MATERIAL, SAID STAINLESS TUBE BEING ELECTRICALLY INSULATED FROM SAID INNER AND OUTER ELECTRODES, IMMERSING SAID STAINLESS TUBE AND SAID INNER AND OUTER ELECTRODES IN AN ELECTROLYTE CONSISTING ESSENTIALLY OF 20 TO 60 WT. PERCENT H2SO4 MAINTAINED AT A TEMPERATURE OF 110* TO 175*F, ESTABLISHING A POTENTIAL DIFFERENCE BETWEEN SAID INNER AND OUTER ELECTRODES, OF A MAGNITUDE SUFFICIENT TO EFFECT THE PASSAGE OF A CURRENT DENSITY OF 2 TO 15 AMPERES PER SQUARE INCH, WHILE CAUSING SAID ELECTROLYTE TO FLOW IN THE ANNULAR SPACES BETWEEN SAID ELECTRODES AND SAID STAINLESS TUBE AT A RATE SUFFICIENT TO SUPPORT SAID CURRENT DENSITY, WHEREBY THE CURRENT IS CAUSED TO PASS SUBSTANTIALLY RADIALLY BETWEEN THE INNER AND OUTER ELECTRODES THROUGH SAID TUBE PROVIDING A FILTRATION SYSTEM INCLUDING AT LEAST ONE ULTRAFFILELECTRODES AND SAID TUBE, AND THE SURFACE OF SAID TUBE FACING THE MORE NEGATIVE OF SAID ELECTRODES WILL BE DISSOLVED AND CONDITIONED.
 2. The method of claim 1, wherein said H2SO4 concentration is between 30 and 50 percent, said temperature is between 120* and 150*F and said current density is between 5 and 10 amperes per square inch.
 3. The method of claim 2, wherein the polarity of said inner and outer electrodes is reversed every 10 to 60 seconds.
 4. The method of claim 1, wherein the polarity of said inner and outer electrodes is reversed every 10 to 60 seconds. 